Variability in biological responses influenced by upwelling events in the Eastern Arabian Sea

This study documents the result of field observations from the southeastern Arabian Sea during onset, peak and late phases of summer monsoon. The pronounced variations in phytoplankton standing stock (chlorophyll a), primary production, meso zooplankton standing stock and physico-chemical variables were examined according to the different stages of upwelling i.e., newly upwelled (Type-1), mature (Type-2) and aged upwelling (Type-3). Type-1 stage was observed along the inshore waters off 8–10°N during the onset of summer monsoon and along north of 13°N during peak and late summer monsoon. This stage was characterized by high wind stress (∼ 0.72 dynes/cm2) and upsloping of isotherms and isolines of nitrate towards the coast. Type-2 stage was identified along the inshore regions of 8–11.5°N during peak summer monsoon with relatively stable and nutrient rich (> 2 µM NO3) cold (> 27.7°N) surface waters. Type-3 stage was observed along 8°N transect during the late summer monsoon, which was characterized by relaxed wind stress. Upwelling signatures were not prominent along this transect. Relatively higher biological production was observed during Type-2 stage. Distribution of meso zooplankton biomass followed the same trend as that of chlorophyll a and primary production. Northward propagation of upwelling was observed as summer monsoon proceeded. Spatial variations in biological production were observed in the Arabian Sea as a consequence of the variations in the intensity of upwelling.